Among secondary batteries currently used in application, lithium ion batteries developed in the early 1990s are small, light, and large-capacity batteries and have been widely employed as power sources for portable devices since the advent of 1991.
Lithium secondary batteries are being spotlighted in the merits of having their superior energy density and higher operating voltage than conventional batteries such as Ni-MH, Ni—Cd, and sulfuric acid-lead batteries using aqueous electrolytes. Especially, in recent years, many studies about power sources for electric vehicles in hybrid types with internal combustion engines and lithium ion batteries are actively underway in the United States, Japan, and Europe.
While lithium ion batteries are considered as large-scale batteries in use for electric vehicles from the view of energy density, nickel-hydrogen batteries are used until now in the reason of stability. With respect to lithium ion batteries to be used for electric vehicles, the most urgent problem is a high price and stability. Specifically, there is generation of abrupt structural deformation if a positive active material, such as LiCoO2 or LiNiO2, which is currently used in commercialization, is heat up at 200 to 270° C. in an overcharge state of the battery. Such structural deformation results in a shortness that causes a discharge of oxygen from a lattice and thereby a crystalline structure becomes unstable due to secession of lithium and much severe degradation of thermal characteristics.
For eliminating the shortness, they are trying to prevent an abrupt exothermic reaction or to shift heat starting temperature toward high temperature by partly replacing nickel with a transition element. A material such as LiNi1-xCoxO2 (x=0.1˜0.3), where nickel is partly replaced with cobalt, shows excellent charge/discharge and lifetime characteristics, but insufficient to solve the problem of thermal stability. Additionally, there are much known technologies for organizing and manufacturing an oxide composite of Li—Ni—Mn series where a Ni position is replaced partly with Mn that is excellent in thermal stability, or of Li—Ni—Mn—Co series where an Ni position is replaced with Mn or Co. Japanese Patent No 2000-227858 has recently disclosed a new concept of positive active material for making a solid solution by uniformly dispersing a compound of Mn and Ni in atomic levels without the partial replacement of a transition metal in LiNiO2 or LiMnO2. For example, according to European Patent No. 0918041 or U.S. Pat. No. 6,040,090 about composition of an Li—Ni—Mn—Co series oxide composite where Ni is replaced with Mn and Co, LiNi1-xCoxMnyO2 (0<y≤0.3) has thermal stability more improved than a material which is formed of Ni and Co only, but is insufficient to solve the problem of thermal stability involved in Ni series materials.
In the meantime, methods for varying surface composition of positive active materials in contact with electrolytes are applied to solve such a problem. One of the methods is to coat the surface. Generally, an amount of coating is known as equal to or smaller than 1 to 2 weight % compared to a positive active material and a coated layer is known as restraining a side reaction with an electrolyte by forming a very thin film layer about several nanometers. In some case, high temperature of thermal treatment after coating forms a solid solution on the surface of powder particles to make the inside of the particles different from the surface in metallic composition. In this case, a surface layer coupled with a coating material is known as having a thickness equal to or smaller than several tens nanometers and its coating effect becomes lower when it is used for a long term of several hundreds cycles because of a sharp composition difference between the coated later and the bulk of particles.
Additionally, the coating layer loses half its effect due to incomplete coating of ununiform distribution over the surface.
For eliminating such shortness, Korean Patent Application No. 10-2005-7007548 has proposed a solution about a lithium transition-metal oxide having a concentration gradient of metallic composition. However, this method is able to make inner and outer layers different each other in metallic composition, but the metallic composition does not vary continuously and gradually in a generated positive active material. Although it is possible for the method to form a gradual concentration gradient of metallic composition through thermal treatment, there is merely generated a difference of concentration gradients due to thermal diffusion of metal ions at high temperature of thermal treatment equal to or higher than 850° C. Additionally, a powder synthesized by the foregoing invention is unsuitable for a lithium secondary battery positive active material due to low tap density of the powder because it does not use ammonia which is a chelating agent. Additionally, this method fails in reproducibility due to difficulty of lithium amount control in an outer layer in the case of using a lithium transition metal oxide as an inner material.
Korean Patent Application No. 10-2004-0118280 proposes a double layer structure having a core-shell structure. This foregoing invention reports a material having high thermal stability and high capacity characteristics by organizing a positive composite of high capacity characteristics in a core, through a CSTR reactor, and by organizing a positive composite of high thermal stability in an outer shell.
However, even with the forgoing invention, it is difficult to form a continuous concentration distribution between two interfaces due to diffusion of metal ions at an interface where the inner core meets the outer shell.